#1,182 – Using RenderSize Properties in Custom Elements

When writing rendering code for a custom element that derives from FrameworkElement, you can use the ActualWidth and ActualHeight properties to know how to render the element.  These properties indicate the desired final size of the element, after all layout calculations have been done.

    public class MyFrameworkElement : FrameworkElement
    {
        protected override void OnRender(DrawingContext dc)
        {
            dc.DrawLine(new Pen(Brushes.Blue, 2.0),
                new Point(0.0, 0.0),
                new Point(ActualWidth, ActualHeight));
            dc.DrawLine(new Pen(Brushes.Green, 2.0),
                new Point(ActualWidth, 0.0),
                new Point(0.0, ActualHeight));
        }
    }

If a custom control derives from UIElement, it won’t have access to the ActualWidth and ActualHeight properties, but can instead use RenderSize.Width and RenderSize.Height.  (You can also use these properties from within an element that derives from FrameworkElement, since FrameworkElement inhertis from UIElement).

    public class MyUIElement : UIElement
    {
        protected override void OnRender(DrawingContext dc)
        {
            dc.DrawLine(new Pen(Brushes.Blue, 2.0),
                new Point(0.0, 0.0),
                new Point(RenderSize.Width, RenderSize.Height));
            dc.DrawLine(new Pen(Brushes.Green, 2.0),
                new Point(RenderSize.Width, 0.0),
                new Point(0.0, RenderSize.Height));
        }
    }

#1,145 – Using RenderSize in Custom Shape

When drawing a geometry in a custom Shape element, you could draw using hard-coded coordinates.  It’s more common, however, to use the RenderSize property of the UIElement to render the object so that the geometry scales based on the size of the control.

Below, we create a custom shape that draws a diagonal line from the upper left corner of the control to the lower right.

    public class MyShape : Shape
    {
        protected override Geometry DefiningGeometry
        {
            get
            {
                double maxWidth = RenderSize.Width;
                double maxHeight = RenderSize.Height;

                StreamGeometry geom = new StreamGeometry();
                using (StreamGeometryContext ctx = geom.Open())
                {
                    ctx.BeginFigure(
                        new Point(0.0, 0.0),
                        false,
                        false);
                    ctx.LineTo(
                        new Point(maxWidth, maxHeight),
                        true,
                        false);
                }

                return geom;
            }
        }
    }

We can use the shape in XAML as follows:

    <StackPanel>
        <loc:MyShape Stroke="Black" StrokeThickness="1"
                     Height="50" Width="50"
                     HorizontalAlignment="Center"/>
    </StackPanel>

Now when we change the size of the underlying control, the geometry adjusts as well.

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#757 – RenderSize, ActualHeight and ActualWidth

The ActualHeight and ActualWidth properties of an element that inherits from FrameworkElement indicate the final height and width of the element, after layout has taken place.

This same information is also available via the UIElement.RenderSize property (UIElement is the parent class of FrameworkElement).  RenderSize is of type Size, which is a struct containing the width and the height.